SLIDE 1 Jiří PŘIBIL1, Anna PŘIBILOVÁ2, Ivan FROLLO1
1Institute of Measurement Science, Slovak Academy of Sciences,
Dúbravská cesta 9, SK-841 04 Bratislava, Slovakia.
2Institute of Electronics and Photonics, Faculty of Electrical
Engineering & Information Technology, Slovak University of Technology, Ilkovičova 3, SK-812 19 Bratislava, Slovakia.
1 5th International Electronic Conference on Sensors and Applications, 15–30 November 2018
Table of contents:
- Introduction – motivation of the work
- Description of the investigated open-air MRI device
- Measurement and signal recording experiments
- Visualization and statistical analysis of signal energy properties
- Discussion of obtained results
- Summary and Conclusion
Analysis of Energy Relations between Noise and Vibration Signals in the Scanning Area of an Open-Air MRI Device
SLIDE 2
Mechanical vibration causing image blurring of thin layer samples produces also acoustic noise degrading the recorded speech signal during MR scan of the human vocal tract. The acoustic noise has also negative physiological and psychical effects on the exposed person depending on the noise intensity and time duration of noise exposure. In order to minimize these negative factors, this work is focused on mapping of the energy relationship between vibration and noise signals measured in the MRI scanning area and its vicinity. MR imaging is accompanied with vibration due to rapidly changing Lorenz forces in gradient coils producing significant mechanical pulses during execution of a scan sequence.
Motivation of Our Work
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SLIDE 3 Open-air MRI equipment Esaote Opera:
- stationary magnetic field with B0 = 0.178 T is produced by a pair
- f permanent magnets,
- gradient system consists of 2 x 3 planar coils situated between
the magnets and an RF receiving/transmitting coil with a tested
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Basic Description of the Investigated MRI Device
SLIDE 4
An examined person in the MRI device during scanning of the vocal tract: (1) a patient’s bed with the tested person, (2) a pick-up microphone, (3) a head RF coil, (4) an external RF pre-amplifier.
MR image of the vocal tract in a sagittal plane using the SSF-3D scan sequence (above); parallel recorded speech signal with an acoustic noise (below).
Example of MR Scan of the Human Vocal Tract
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SLIDE 5 Practical realization of experiments consists of 3 phases:
1) Preliminary mapping of the acoustic noise SPL distribution in the MRI device vicinity at distances DX = <45 ~ 90> cm. 2) Real-time recording
vibration and noise signals during execution of a scan MR sequence and manual measurement of noise SPL for:
- different MR scan sequences of Hi-Res and 3-D type,
- {Coronal, Sagittal, and Transversal} orientation of scan slices,
- times TE={18, 22, 26} ms, and TR={60, 100, 200, 300, 400, 500} ms,
- different of object/subject masses inserted in the MRI
scanning area (testing phantom with a weight of 0.75 kg and male / female person with a weight of 85 kg / 55 kg). 3) Off-line processing of vibration and noise signals:
- calculation of the signal energy parameters based on RMS,
Teager–Kaiser energy operator, cepstrum, and autocorrelation; histogram building, statistical analysis, visualization.
Description of Performed Experiments
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SLIDE 6
Placement of sensors in the open-air MRI device for recording of vibration, noise, and SPL measurement; the water phantom inside the knee RF coil.
Arrangement of Measurement and Signal Recording in the Open-Air MRI Device
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SLIDE 7 Real-time parallel recording of the vibration and noise signals in the scanning area of the MRI device was using:
- the multi-function environment meter Lafayette DT 8820 – placed at
the distance DX = 60 cm from the central point of the scanning area, at the height of 75 cm from the floor – for noise SPL measurement,
- the vibration sensor SB-1 mounted on the surface of the plastic
holder of the bottom gradient coils,
- the 1" Behringer dual diaphragm condenser microphone B-2 PRO
with a cardioid pickup pattern for noise signal recording,
- the mixer device Behringer XENYX 502, as a part of the Behringer
PODCAST STUDIO equipment for pre-amplifying and processing
- f input analog signals,
- manual synchronization of signal recording and the MR scan
process by the console operator,
- the sampling frequency of 32 kHz (resampled to 16 kHz) – signals
were next processed by the sound editor program Sound Forge 9.0a.
Conditions of Measurement and Signal Recording
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SLIDE 8 The vibration sensor constructed primarily for acoustic pickup in musical instruments: contains a piezoelectric element
- n a brass circular target,
can be used in the stationary magnetic field with low B0. Our frequency range of interest corresponds to the frequency range of a voiced speech <10 Hz ÷ 3.5 kHz> and bass instruments: the sensor SB-1 with a 1’’ brass disc was finally used, designed primarily for contrabass pickup.
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5 10 —› fexc [Hz] —› 20*log(Bv/Bv0) [dB] USB-1 fref
200 400 600 800 1000 1200 10 20 30 40 —› Uexc [mV] —› Ba [mV/ms-2] USB-1 UexcBa0
The Vibration Sensor Usable for Measurement in the Low Magnetic Field Environment
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SLIDE 9 The vibration sensor SB-1 is mounted in the left corner
- n the bottom plastic cover in both cases.
Recording of Vibration in the MRI Opera
Two working arrangements for vibration signal recording:
a) with a lying testing person b) with a spherical water phantom
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SLIDE 10 MR Scan Sequences Used in Experiments
Type Name of sequence TE [ms] TR [ms] FOV 1 Matrix size Hi-Res SE 18 HF 18 500 250x250x200 256x256 Hi-Res SE 26 HF 26 500 250x250x200 256x256 Hi-Res GE T2 22 60 250x250x200 256x256 3-D SS 3D balanced 5 10 200x200x192 200x200 3-D 3D-CE 30 40 150x150x192 192x192
Description of used MR sequences and their basic scan settings.
1 In all cases the sagittal slice orientation and the slice thickness of 4.7 mm were pre-defined.
The TE and TR parameters were set manually, according to basic values introduced in this table to perform measurement and comparison in the range enabled by the current sequence.
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SLIDE 11 SPL values with background SPL0 (left), basic statistical parameters (right).
Measurement conditions and limitations:
- The SPL meter at the height of 75 cm from the floor (level of the bottom
gradient coils), and at 30 degrees from the MRI left corner.
- The minimum DX = 45 cm was set to eliminate interaction of metal parts of
the SPL meter with static magnetic field of the MRI device.
- The maximum distance DX = 90 cm was chosen with respect to the fact that
the SPLs measured at this position are close to the background noise SPL0
Obtained Results of Noise SPL Measurement
40 50 60 70 80 90 100
—› DX [cm]
50 55 60 65 70 75 80
—› Noise SPL (C)[dB]
SE-TR18 GE-T2-22 SPL0
SPL_0 SE-TR18 GE-T2-22 55 60 65 70 75 80
—› Noise SPL (C)[dB]
Mapping of acoustic noise SPL at distances DX of {45, 50, 55, 60, 70, 80, and 90} cm for SE and GE types of Hi-Res scan sequences:
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SLIDE 12
From left to right: signal RMS together with SPL values, bar-graphs of basic statistical parameters of Enc0 values, corresponding histograms for Enc0 parameter. Used sequences of {Hi-Res SE-HE, Hi-Res SE-HF, Hi-Res GE-T2, SS-3Dbal, 3D-CE}, in all cases with a sagittal slice orientation.
Visualization of Energetic Relations of Recorded Vibration and Noise Signals
1) For different sequence types:
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SLIDE 13 From left to right: bar-graph of signal RMS values, histograms of Enc0, mutual Fv1 / Fv2 positions.
Used Hi-Res SE scan sequences (TE=18 ms, TR=500 ms).
Visualization of Energetic Relations of Recorded Vibration and Noise Signals
2) For different slice orientations of {coronal, sagittal, transversal}:
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—› Signal RMS [-] —› Fv2 [Hz] —› Rel.occurence [%]
SLIDE 14
Visualization of Energetic Relations of Recorded Vibration and Noise Signals
3) For different TE times of {18, 22, 26} ms:
From left to right:
– bar-graph of signal RMS values, mean mutual Fv1 / Fv2 positions; – bar-graphs of basic statistical parameters of: EnTK, Enc0, Enr0.
Used Hi-Res SE-HF sequences (TR=500 ms, sagittal orientation).
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SLIDE 15 From left to right: a) signal RMS together with noise SPL values, mean Enc0, mean Enr0, mean EnTK; b) statistical parameters of Enr0 values: standard deviation, relative maximum, range. Used Hi-Res GE-T2 sequences with TE=22 ms, and sagittal orientation.
—› SignalRMS [-]/SPL [dB] —› Enc0 [-] —› EnR0 [-] —› EnTK [-]
4) For different TR times of {60, 100, 200, 300, 400, 500} ms:
—› Enc0 [-] —› Enc0 [-]
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Visualization of Energetic Relations of Recorded Vibration and Noise Signals
SLIDE 16 Obtained Results for Different Subjects in the Scanning Area of the MRI Device
Subject type 1 Vibrations (SB-1) RMS EnTK Enc0 Enr0 Water phantom 34.6 4.69 0.0380 24.0 Male 26.8 4.96 0.0404 14.4 Female 28.7 4.93 0.0402 16.6
1 Used Hi-Res SE-HF scan sequences with TE=18 ms, TR=400 ms, sagittal orientation.
Subject type 1 Noise (Mic. B2-Pro) RMS EnTK Enc0 Enr0 Water phantom 20.1 4.05 0.0255 8.5 Male 25.5 4.51 0.0328 15.9 Female 23.2 4.19 0.0286 10.6
5) For different object/subject masses in the MRI scanning area:
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SLIDE 17
From analysis of setting of the MR scan parameters follows: 1. Influence of the choice of slice orientation on the energy of the produced vibration and noise signals is significant:
→ the maximum can be found in the sagittal plane and the minimum in the transversal plane.
2. Experiments confirm influence of TR and TE times on the vibration and acoustic noise properties:
→ the TR parameter determines the fundamental frequency FV0 and its stretching causes energy fall of the final signal, → TE time affects higher frequencies (the first two dominant frequencies FV1,2) but the signal energy is unaffected.
3. Spectral differences between two mostly used MR scan sequences (SE/GE types) show that:
→ the GE sequence has more structured noise, → the SE sequence generates more compact vibration with higher energy in the final effect.
Discussion of Obtained Results I.
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SLIDE 18 Further investigations show that:
4. Energetic relations of vibration and noise signals for different sequence types manifest small differences:
→ the 3-D SE sequence produces the noise with minimal intensity, maximum can be found for the SS-3Dbal (TE=5 ms, TR=10 ms).
5. Performed experiments confirm that the produced vibration and acoustic noise are principally influenced by a load of a person lying in the scanning area of the MRI machine:
→ the energy of the vibration signal is significantly higher for the male/female persons lying inside the MRI scan area in both types of analyzed MRI devices than using phantoms only, → the situation is opposite for noise signals – the maximum corresponds to the scanned male person (the greatest volume). → vibration signals with a testing person have smaller dispersion
- f spectral decrease, but higher frequency of the spectral
centroid and higher FV1 to FV2 mutual positions.
Discussion of Obtained Results II.
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SLIDE 19 1) The maximum of acoustic noise SPL of about 78 dB(C) was achieved at the distance of 45 cm from the central point of the MRI scanning area for the GE scan sequence with short TE and TR times and sagittal slices orientation:
- no special hearing protection aids (ear plugs or ear muffs) are
necessary for the examined persons lying in the scanning area of the investigated MRI device Esaote Opera.
- Due to low scanning times for mostly used 3D or Hi-res
sequences (less than 15 minutes), exposition of the human
- rganism and its ear by the noise and vibration is not great.
2) The obtained results will serve to create databases of initial parameters (such as the bank of noise signal pre- processing filters) for acoustic noise suppression in parallel with speech recording applied for 3-D modeling
Summary & Conclusion
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SLIDE 20
Funding
This work has been supported by: the Grant Agency of the Slovak Academy of Sciences (VEGA 2/0001/17), the Ministry of Education Science, Research, and Sports of the Slovak Republic VEGA 1/0905/17.
Future plans:
– measurement of the vibration signal on the surface of both plastic holders for knowledge about the contribution of the upper gradient coil to the resulting produced noise, – additional measurement and experiments for better description of the acoustic noise conditions in the scanning area and in the vicinity of the MRI device.
